Titanium components (defined as equipment and components of equipment made from, coated with or clad with titanium metal or otherwise having a surface of titanium metal) are useful in a variety of systems, processes and environments in which corrosion resistance is important. For example, titanium metal is used as a material of construction or fabrication for reaction vessels, liners and other reactor internals in chemical and other industrial processes that use or involve exposures to corrosive materials (including solvents, reactants, and by-products) or environments, such as an oxidizing environment. Titanium components also include heat exchangers due to titanium's corrosion resistance and resulting extended life of the equipment. Titanium components in the form of packing materials are often used in distillation columns and other separation devices used for gas-liquid separations involving corrosive materials. Although titanium components themselves are corrosion resistant, iron oxide deposits may form on surfaces of such titanium components when they are employed in systems or processes in which they come in contact with a source of iron, such as soluble iron or iron compounds which may be present as impurities in process streams. For example, titanium components are often integrated into systems which also employ relatively inexpensive stainless steel materials in areas where corrosion resistance is less important. These stainless steel materials may introduce dissolved iron into the system, which can accumulate on the surface of titanium components as iron oxide. In other systems, possible sources of iron include catalysts, liquid process streams and impurities.
The accumulation of iron oxide on a titanium component surface is often gradual but amounts of iron oxide accumulated over time can affect the usefulness of the titanium component. In the case of a titanium component in the form of a heat exchanger, for example, the accumulation of iron oxides on the surface of the titanium component may interfere with heat transfer. When titanium components are used in a distillation column as packing materials, a thin layer of iron oxides may form on the surface of the titanium packing when repeatedly exposed to dissolved iron, and it has been reported that “Accumulations of iron oxide . . . on titanium structured packing can promote or accelerate combustion of titanium. It may be appropriate to periodically remove accumulations of such materials through chemical or other means. However, if removal is accompanied by loss of titanium, it can create thinner metal, which may be sensitive to ignition” (Centerline, Vol. 5, No. 2, Summer 2001, pp. 6–8, 15–18, Mary Kay O'Connor Process Safety Center). This publication also reports that the presence of iron oxides “accelerated the oxidation of the titanium [packing] via a mechanism known as the Thermite Reaction in which the oxygen for combustion is taken from a less reactive metal oxide.” Examples of thermite type reactions involving titanium metal and iron oxides may be represented by the following: 2FeO+Ti→TiO2+2Fe; or 2Fe2O3+3Ti→3TiO2+4Fe.
Methods and compositions for removing oxide deposits are known from U.S. Pat. No. 3,957,529, which discloses a cleaning solvent comprising sulfuric acid and citric acid and its use to clean metal surfaces; U.S. Pat. No. 4,174,290, which discloses a method for removing metal oxides with a composition comprising an amine, a strong mineral acid and citric acid; and U.S. Pat. No. 4,250,048, which discloses a method for removing metal oxides with a composition comprising an ammonia derivative such as an amine, a strong mineral acid and an organic chelating agent for the metal oxides in an aqueous solution with a pH of about 0.5–3.0. U.S. Pat. No. 4,623,399 discloses a method of removing iron oxide scale from metal surfaces with a composition comprising a hydroxyethyl ethylenediamine triacetic acid and an organic acid such as formic acid. It also has been reported that ferrometal corrosion products are removed by use of citric acid or citric acid-tannin complexing agents with erythorbic acid reducing agents.
Although traditional methods and compositions are often useful for removal of iron oxides from various surfaces, their utility in particular applications can be limited in various respects, such as inadequate selectivity to iron oxides over other desirable metals that may be present and lack of compatibility or difficult integration with other aspects of the application. In the case of iron oxide deposits on surfaces of titanium components, selective removal, such that iron oxide is removed without substantial damage to or dissolving of the titanium component, can be especially important due to the relatively high cost of the titanium components.
It would, therefore, be desirable to provide a method and composition for removing iron oxide deposits from a titanium component without damaging the titanium component. In the case of titanium components used in chemical and other industrial processes, it would be particularly desirable to provide for removal or control of surface deposits of iron oxide on titanium components integrated with other aspects of the processes.